But the truth is that we are in the early days of understanding what’s in breastmilk and how it functions.

Last week Spanish researchers announced that they have succeeded in creating a map of the “microbiome” of human milk, identifying the bacterial species present in colostrum and mature milk. They did this through a proceess called DNA pyrosequecning. A key finding: There are are over 700 bacterial species present in human milk.

But human milk has a distinct purpose and has evolved over thousands of years to meet the needs of human infants. And for that reason its components read a little like a backwards map of infant nutritional and immunological needs. The scientists’ findings are the answer, now we just have to figure out what the question is.

This finding has spurred researchers to wonder whether the bacteria in breast milk play a metabolic role or an immune one. Do the bacteria help infants digest breast milk or do they help babies distinguish between beneficial and foreign invaders?

Just in case you got a little queasy hearing about all the bacteria in your milk (we wouldn’t blame you given headlines like “700 bug species in breast milk!“) it’s important to note that bacteria are necessary for the healthy functioning of our bodies. And it’s clear that human milk is instrumental in creating a healthy world of bacteria in our babies’ guts. So when you hear ‘bacteria’ in this context think “good guys.”

More interesting than the sheer number of bacterial species found in milk is the variation the researchers found when comparing milk from mothers who had different weights and birth experiences:

Mothers who were overweight or gained more weight in pregnancy had less bacterial diversity.

Mothers who gave birth by planned cesarean section had less bacterial diversity.

Mothers who gave birth by unplanned cesarean section (had experienced some labor) had bacterial diversity similar to those who had given birth vaginally, suggesting a hormonal effect on milk composition.

As babies became older, the bacterial species present in milk changed.

We are a long way off from being able to read the backwards map of breastmilk, but this study is a strong step in that direction.

Photo: This is not a picture of bacteria in milk, but of cells in my milk, taken by the lab of Dr. Kathleen Arcaro for breast cancer research.